wfc3fandomcom-20200215-history
UVIS Channel
Introduction The WFC3 UVIS channel, named due to its UV and visible light detecting capabilities, is one of two detectors contained in WFC3. The UVIS detector consists of two connected CCDs, an optical train, filter selecton mechanism, and shutter mechanism. diagram The CCDs are cooled by a four-stage thermoelectric cooler. Channel Specifications The CCD detector consists of two 2051 x 4096 chips with 15 x 15 μm square pixels. The two chips are butted together with a 35 pixel gap between chips (~1.4 arcsec). The TEC cools the detector to -83°C, and also includes a second cooled window to reduce radiative heat. CCD Basics A CCD (charge-coupled device) is a Silicon detector with a 2-D array of summing wells, or pixels. Each pixel accumulates electric charge proportional to the number of photons striking the detector at that location. As the pixels accumulate charge, electric fields are formed and create additional wells at depletion regions (___What are depletion regions?___). The size of the depletion regions is controlled by the voltage of three gates (___what are these gates?___). The end of an exposure is controlled by changing the voltage of the gates - packets of charge sequentially transfer to the adjacent pixel until they reach the readout circuitry. (___Find more info. and clarify this. ____) Problems and Solutions Differences between WFC3 CCDs and ACS/WFC CCDs: The WFC3 CCDs are optimized in the UV wavelength range (200-400 nm), (___...while ACS...___). WFC3 CCDs also have lower readout noise, and charge-injection capability (___get more info from handbook on this___). The WFC3 CCDs have a 35 inter-chip gap instead of a 50 pixel gap in ACS/WFC. Full-frame Readout The WFC3 CCDs have four total amplifiers, two for each chip. This allows for faster readout, and thus reduced overhead time. Note that raw images contain more pixels than the CCD chips have due to detector overscan pixels. These overscan pixels are not exposed to light, and are instead used to characterize detector performance. There are four different types of overscanning - serial and parallel, which can be implemented as either physical or virtual. Serial overscanning is when a fixed number of unexposed pixels at each end of the serial (___?___) shift register. Parallel overscanning is extra parallel shifting before or after exposed rows have been read out. Physical overscanning is a characteristic of detector hardware, while virtual overscanning is a software function. (___add more & clarify___). Subarrays (___what are subarrays and why are they useful___) Rectangular subarrays can be used to read out only certain parts of the CCD if desired, useful for reducing overhead time. Subarray images contain no virtual overscan data, and serial physical overscan is present only if defined (__?? add more, delete?___) (___What are the predefined subarrays? Policy on custom subarrays?___) On-Chip Binning (___What is binning and why is it useful___) Several adjacent pixels may be read out as a single pixel - either 2x2 or 3x3. Overscan geometry is complicated by the need to truncate "odd" pixels, so each half of a row must be considered separately. Both binning types include enough overscan pixels to adjust the image for bias subtraction. In 2x2 binning, the binned array size is 2070x2102. In 3x3 binning, the binned array size is 1380x1402. (___maybe add a diagram including overscan?___) Quantum Efficiency Quantum Efficiency (QE) refers to the detector, while throughput refers to optics. The QE of WFC3's CCDs is lower than that of ACS/WFC - 70% max compared to 85% max. graphs Multiple-Electron Events at Short Wavelengths In Silicon, photons of energy over 2.1 electron-volts (eV) can produce multiple electron-hole pairs. "Quantum yield": at higher energies, incident photons can extract more than one electron from the valence band (___valence band?___). This effects noise level, and is compensated for in calculations of ETC equations. Flat Fields Before launch, ground-based flat field images were taken using each different filter, achieving a S/N ratio of ~200/pixel. In flight observations of a rich stellar field with large scale dithers will be used to refine ground flat field images. The resulting flat field images support photometry to ~1% accuracy. img Long-Wavelength Fringing Fringing is caused by interference due to multiple reflections between front and back surfaces of the CCD. Amplitude (of the fringes?) is strongly affected by detector thickness, wavelength, and spectral resolution. Fringing can be corrected if an appropriate flat field image is available. The fringe pattern can also be modeled by interpolating between or combining monochromatic patterns previously obtained, or from theoretical calculations. The WFC3 CCDs exhibit fringing at wavelengths longer than 750 nm. The peak-to-peak variation in brightness of flat-field signal increases with wavelength (over 750 nm), and can reach ±50% at the longest wavelengths. Linearity and Saturation When CCDs are overexposed, blooming occurs. Blooming is when a full pixel spills excess photo-generated charge into adjacent pixels along its column. Extreme overexposure isn't believed to cause any long-term damage to the CCDs - in other words, there are no bright-object limits. (___What other instruments have permanent damage due to blooming?___) Dark Current Predecessors to the WFC3 CCDs worked with surface channels, creating significant charge traps which limit CTE and (increasing?) dark current. (___What are surface channels?___) The WFC3 CCDs use buried channels. In buried channel devices, a shallow Silicon layer is implanted below the surface to store and transfer the collected signal charge away from the traps at the interface. Dark current in WFC3 is addtionally reduced using MPP technology, and depends on two factors: the density of interface states (__?__) and the density of free carriers (holes and electrons) that populate the interface. MPP technology is a type of (___?___) Bad Pixels Hot pixels depart from the average dark current by > 100 times the standard devation, and are identified using one hour dark current frames. Dead pixels remain below 1000 electrons (mostly near 0), and are measured with flat-field illumination of ~50% full well. Non-functional pixels make up 0.65% of the total array. Cosmic Rays WFC3's similarity to ACS/WFC allows us to predict the effects cosmic rays will have on the camera. Whether or not a pixel is affected depends on charge diffusion, so different areas do not affect the percentage of affected pixels (__clarify___). As a reference, ~1.5-3% of ACS's pixels were affected in a 1000 second exposure. Charge-Transfer Efficiency CTE is a measure of how effective the CCD is at moving charge from one pixel location to the next without reading out the chip. Small traps in the silicon lattice compromise this process by retaining electrons and releasing them late. For small signals, it can have a large effect. The CTE of the WFC3 CCDs is ~3-5 x 10e/transfer.